1. Field of the Invention
Two-component (2K) silicone compositions have been known for a relatively long time in the prior art and are often used as adhesives and sealants in various applications.
2. Description of the Related Art
Two-component silicones which crosslink at room temperature are referred to as “room temperature vulcanizing 2 part silicones” (RTV-2). One of the two components is frequently referred to as the polymer composition or as the “A component.” The second component is frequently referred to as the hardener composition or else as the “B component.” Such formulations typically contain tin compounds as crosslinking catalysts which catalyze the crosslinking (polycondensation) of the polymers by means of the crosslinker components at room temperature, as described, for example, in Walter Noll, Chemistry and Technology of Silicones, 1968, 2nd edition, page 395ff.
Although the polycondensation reaction can also proceed without a catalyst, catalysts are required to allow the crosslinking to occur within acceptable time frames. Such catalysts are usually based on heavy metals, usually organotin compounds, as previously indicated. Discussion has heightened recently regarding the risk potential of such organotin compounds and also the known ability of tin compounds to promote the backreaction (reversion) making it necessary to seek alternative catalysts. However, previous attempts at a solution have shown that the reactivity of alternative catalysts is not sufficiently good or else the composition is not storage-stable. Thus, for example, bismuth carboxylates in combination with aminofunctional alkoxysilanes tend to form of a colloidal Bi(0) precipitate, resulting in a decrease in the amount of active Bi(III) available, and destroying storage stability.
Titanium-catalyzed, one-component condensation compositions are known in the prior art, as described, for example, in WO 01/49774 A2, but cannot be applied to two-component systems since titanium compounds lead to spontaneous vulcanization of α,ω-dihydroxy-functional polymers used in such compositions. In addition, titanium compounds are greatly inhibited by amine-containing compounds, which rules out the formulation of self-adhesive compositions.
Other metal compounds such as zinc acetylacetonate or aluminum acetylacetonate when used as catalysts lead to functionalization of the reactive end groups, i.e. a reaction competing with crosslinking, which brings about a conversion to the RTV-1 mechanism.
As tin-free alternatives, calcined kaolin is used as catalyst in WO 2009/080266, and in EP 0 933 398 A together with basic components. Since kaolin is present in dispersed form and is used in WO 2009/080266 in amounts of from 3 to 400 parts per 100 parts of polymer, it has the disadvantage that no transparent compositions cannot be produced.